In the manufacture of products comprising the continuous extrusion of thermoplastic materials, and particularly when sequentially extruding multilayered articles such as many common types of insulated electrical wires and cable, the heat-softened and often as yet semitacky thermoplastic materials passing under forming pressures in contact with the metal faces of guides, dies or other components of the extrusion apparatus, are frequently "pulled" or extended by their transient adherence to the metal surface. The "pulling" or extending of the plastic material due to its transient or intermittant adherence causes protrusions or marring of the surfaces of the plastic undergoing shaping, notwithstanding polishing and/or plating of the metal faces with relatively smooth impervious finishes such as chrome. This commonly encountered shortcoming in processes of continuously molding thermoplastic materials is especially troublesome in sequential or tandem and multiple head extrusion operations, such as shown in U.S. Pat. Nos. 3,569,610 and 3,646,248, and British Pat. No. 967,567 of Aug. 26, 1964, wherein two or more layers or plies of plastic materials are sequentially formed about a metallic conductor core in the manufacture of electrical wire or cable. These manufacturing operations typically comprise, for example, applying or forming under pressure a subsequent overlying layer of thermoplastic material directly upon a previously formed layer of yet thermoplastic material wherein the previously formed thermoplastic material is pulled through a close-fitting guide orifice of an extruder apparatus into an area under the extruding pressures of the subsequent thermoplastic material whereupon the superimposed composite of the previous layer and forming layer of both thermoplastic materials is then forced through a die.
Moreover, the deleterious effects of this occurrence is far more serious than simply producing irregular or rough surface conditions marring the exterior face of extruded products. For instance, the occurrence of transient adherence and pulling of an inner or underlying layer due to contact with an extruder guide orifice or with a previous extruder die in a sequential extrusion operation forming a multilayered product causes protrusions on the surface of the inner or underlying layer and in turn an irregular interface between the superimposed layers. This occurrence is especially detrimental in the manufacture of electrical wire and cable because the protuberances thus formed on the outer surface of an inner layer, and in turn at the interface between component layers of a composite insulation covering, are subject to dielectric strength failures.